The present invention relates to a near-field optical probe utilizing near-field light to detect an interaction with a microscopic region on the surface of a solid substance, for observation of structural or optical information about a microscopic region less than an input light wavelength or observation of a solid substance surface with high resolution, or utilization for high density information recording and reproduction.
The known probes with a high resolving power utilizing near-field light have been used in near-field optical microscopes and near-field optical heads. Near-field light is produced through a probe tip and interacted with a microscope sample or recording medium, causing propagation light. The detection of such propagation light provides a spatial resolving power exceeding a diffraction limit of light. There also is a method of detecting, by a probe, near-field light caused as a result of an interaction between an incident propagation light and a sample or recording medium. Based on such a principle, the near-field microscope has achieved a resolving power exceeding a diffraction limit of the conventional optical microscope. Meanwhile, the utilization of such a near-field optical probe in a near-field optical head makes feasible data recording with a density surpassing the density on the conventional optical disc.
The applications of near-field light due to the near-field microscope includes one scheme (illumination mode) which brings a probe microscopic aperture and a sample surface into a distance of nearly a diameter of the probe microscopic aperture so that propagation light can be incident through the probe and toward the probe microscopic aperture, thereby producing near-field light in the microscopic aperture. In this case, the produced near-field fight and the sample surface interact with each other, thereby causing scattered light. The scattered light containing an intensity or phase reflecting a sample surface fine structure is detected by a scattered light detection system, thus making feasible high resolution observation previously unachievable by the conventional optical microscope.
Meanwhile, studies have been made on an information recording/reproducing apparatus utilizing near-field light mentioned above.
Many of the today""s information reproducing apparatuses use magnetic or optical discs as information recording mediums to reproduce information. In particular, CDs are one kind of optical discs being utilized as mediums to record a large amount of information because they can record information with high density and are mass produced at low cost. The CD is formed, on its surface, with pits in a size of nearly a wavelength of laser light to be used for reproduction and a depth of about a quarter of that wavelength. Thus, reading can be made utilizing light interference phenomenon.
It is general practice to utilize a lens optical system for optical microscopes in reading recorded information from an optical disc such as by the CD. In the case of increasing the density of information recording by reducing the pit size or track pitch, it is impossible to reduce the laser light spot size smaller than a half of the laser light wavelength due to the light diffraction limit problem. Thus, one runs into a wall in that the information recording unit be impossible to is decreased in size to be smaller than a laser light wavelength.
Besides the optical discs, the magnetic discs recording information by the magneto-optical or phase change recording scheme also have realized high density information recording and producing through a laser light microscopic spot. The information recording density is limited to a spot size to be obtained by focusing laser light.
In order to break through the restrictions imposed by the diffraction limit, there is a proposal of an information reproducing apparatus which uses an optical head having a microscopic aperture with a diameter smaller than a wavelength of laser light to be utilized for reproduction, e.g. approximately {fraction (1/10)}th of the wavelength, to utilize near-field light generated in the microscopic aperture.
The aperture size of a probe is limitative of resolving power where a near-field optical probe is applied as a microscope, and by an information recording density where it is applied as an information processing apparatus. The principle that the near-field optical probe realizes a resolving power or recording density exceeding its light diffraction limit lies in the fact that the light field produced in front of an aperture with a size smaller than a light wavelength contains therein a component having spatially a high frequency (such a component that the light field direction or intensity is different by a slight difference in position) wherein that component interacts with a sample or; recording medium and is scattered into propagation light to be detected. Here, efforts have been made to reduce the aperture size because the resolving power or recording density is improved by increasing the high frequency component within the generated light field.
However, the aperture size is from several tens to several hundreds of nano-meters, and it is difficult to further reduce it in size. This is because the metal film for blocking light leak is deposited with a position control accuracy of about several tens of nano-meters as a size of metal clusters. In order to deposit a film with a sufficient thickness for shading light (approximately 100 nano-meters) around the aperture with the aperture left undeposited, the aperture is difficult to decrease in size smaller than the size currently practiced.
It is therefore an object of the present invention to provide a near-field optical probe which is capable of increasing a high frequency component of near-field light thereby realizing a high resolving power microscope or high recording density information processing apparatus.
In accordance with the present invention, there is provided a near-field optical probe having a microscopic aperture for generating near-field light or scattering and detecting near-field light to interact with an object, characterized by: a fine structure smaller than the microscopic aperture and formed in or nearby a surface of the microscopic aperture.
This structure increases a high frequency component of a light field distribution on the aperture surface without increasing the size of the aperture itself. There is an increase in a spatial high frequency component of produced near-near-field light. As a result, a component containing finer structural information than the conventional one is increased in a light intensity to be observed, thereby realizing a near-field optical probe for a high resolution microscope or high density information recording apparatus.
Preferably, the fine structure s due to an optical characteristic distribution on or nearby the microscopic aperture surface.
This structure provides a variation in optical characteristic distribution regardless of the aperture size, thereby forming the microscopic structure. Thus, realized is a near-field optical probe for a high resolution microscope or high density information recording apparatus.
More preferably, the optical characteristic distribution is due to a roughening shape in or nearby the aperture surface.
This structure is a modification of merely forming a tip form for the optical probe. Thus, a novel effect is obtainable without major change in the manufacture process.
Still more preferably, the optical characteristic distribution may be due to a distribution of a material in or nearby the aperture surface.
This structure facilitates the selection and control of the material to be filled in the aperture and a particle size thereof. Thus, it is possible to obtain a desired resolving power or recording density.
The microscopic aperture may be a bottom portion of an inverted cone-shaped hole formed through a planar substrate.
This structure provides the probe with a planar form, hence achieving a compact apparatus structure. Furthermore, the planar probe can be fabricated by the use of a semiconductor process technology, thus enabling mass production with high reproducibility. Also, where this is applied for an information processing apparatus, it is possible to utilize it as it is with a head floating mechanism such as in a flying head method used in the conventional hard disc.
Preferably, the microscopic aperture is formed in a tip of a sharpened optical waveguide.
This structure can utilize as a probe an optical fiber type probe as used in the conventional near-field microscope, making it possible to effectively apply the technology developed for the near-field microscopes. Also, the probe can be fabricated by using a semiconductor process technology. Thus, the developed semiconductor process technology is effectively applicable.
More preferably, the microscopic aperture is formed in a sharpened protrusion of a cantilever.
This structure can utilize a cantilever type probe used for the conventional near-field microscopes. Thus, the developed semiconductor process technology is effectively applicable.